Polypropylene compositions have gained wide commercial acceptance and usage in numerous commercial applications because of the relatively low cost of the polymers and the desirable properties they exhibit. In general, polypropylene polymers, particularly polypropylene homopolymers, have the disadvantage of being brittle with low impact resistance, particularly at low temperatures. Numerous procedures have been proposed for modifying the properties of the polypropylene homopolymers to improve the impact strength and other low temperature properties. Many, if not most, of such proposals have involved the provision of a propylene/.alpha.-olefin copolymer portion in an otherwise homopolymeric polypropylene. In Liebson et al, U.S. Pat. No. 3,514,501, there is described a process for the production of block copolymers wherein, for example, a prepolymer which is homopolymeric is produced and a block which has at least one other .alpha.-olefin is grown from the prepolymer in a second polymerization step. A second approach to providing improved impact strength comprises the mixing of a polypropylene homopolymer with a propylene/ethylene copolymer. Most commercial products of high impact strength result from the production of a first polymer, usually a polypropylene homopolymer, and the production of a multi-polymeric portion, usually termed a copolymeric portion in the presence of the initial polymer product mixture which still contains active polymerization sites. Whether the resulting product is a block copolymer, a mixture of homopolymer and copolymer or of some other structure is not entirely clear. However, such products are well known in the art, are conventional and have achieved substantial commercial importance. They are often referred to as polypropylene impact copolymers, regardless of their precise structure, and are said to contain a homopolymer phase (often homopolymeric polypropylene) and a rubber phase (the copolymer portion).
Such polypropylene impact copolymers, particularly those wherein ethylene is the other .alpha.-olefin of the copolymer portion, do exhibit improved low temperature impact strength and other improved low temperature properties as well as many of the desirable properties of the homopolymer such as stiffness. However, the impact copolymers have low resistance to stress whitening in situations such as rapid impacting or the bending of fabricated parts and also have poor to moderate toughness. In the case of impact copolymers having a high ethylene content in the copolymer phase, brittle failure is often observed upon impact. These high ethylene content polymers do, however, show better resistance to stress whitening.
The difficulty of obtaining a good balance of properties in a polypropylene composition has been addressed on numerous occasions. In published European Patent Application 208,330 there are disclosed polypropylene compositions said to have improved resistance to stress whitening which comprise homopolymeric polypropylene or peroxide-degraded polypropylene having a grafted ethylene/propylene copolymer portion and, as an additional component an ester derived from a C.sub.12 -C.sub.20 monocarboxylic acid and a polyhydric alcohol. In published Japanese Patent Application 84020522/04 there is described a mixture of polypropylene of specified melt flow and an ethylene/.alpha.-olefin copolymer wherein the molecular weight of the polypropylene has been substantially reduced by treatment with peroxide. Coover et al, U.S. Pat. No. 3,562,790, propose obtaining better properties through the provision of a ternary blend of homopolymers or copolymers and an amorphous copolymer or terpolymer of ethylene, propylene and optionally an unsaturated hydrocarbon of at least one double bond.
In copending U.S. patent application Ser. No. 471,467, filed Jan. 29, 1990, improved resistance to stress whitening is shown by polypropylene impact copolymers having particular ratios of intrinsic viscosities of the copolymer phase to the homopolymer phase.
In Cecchin et al, U.S. Pat. No. 4,734,459, there are disclosed polypropylene compositions comprising a homopolymer portion and a copolymer portion which is crystalline polyethylene and an amorphous ethylene/1-butene copolymer. Care is taken to avoid the presence of propylene in the copolymer portion although provision is made for the presence in the gas phase of a small amount of propylene, e.g., less than 5% by mole on the basis of monomers present. Although these compositions do show some improvement in low temperature properties, the crystallinity in the rubber phase would be relatively high for composition having the higher ethylene contents (in the rubber phase) needed for better stress whitening resistance, and the compositions would be expected to be overly brittle. It would be of advantage to provide improved polypropylene compositions having an improved balance of properties including better low temperature properties such as strength and also good resistance to stress whitening.